The present invention relates to a filter and a process for filtering particle containing or slurry like materials. More particularly, it relates to a depth filter and process for filtering slurries and compositions containing particles and/or gels such as CMP (chemical-mechanical planarization) slurries and photoresist chemicals.
A fluid composition containing a particulate solid component is referred to in the art as a “slurry”. The solid component can be any of a variety of materials including solid particles, cell components, flocculating agents, gel particles or the like. These are found in many applications including photoresist chemicals, biopharmaceutical products and abrasive materials for the semiconductor industry.
Photoresist chemical compositions often contain gels and agglomerates of gels that are formed from the photoresist chemicals due to shear, shock or age of the chemicals. Such gels and agglomerates need to be removed prior to using these chemical compositions
Biopharmaceutical liquid compositions such as cell broths, fermentation liquids, transgenic milks and other transgenic liquids, blood, blood fractions or other bacterial or animal fluids or secretions, contain whole cells, cell components, fats and other solids which need to be removed to in order to further process and recover desired components of these compositions.
Of particular interest are slurry compositions, utilised in CMP to polish wafers in VLSI and ULSI integrated circuit devices. High pH silica CMP slurries are utilised to polish dielectric and polysilicone layers. In addition, acidic silica and alumina or metal/metal oxide abrasive based slurries are utilised to polish metal interconnects. The CMP process uses sub micron (30–500 nm) abrasive particles at a typical concentration of 1–30% by weight.
The typical specification for commercial CMP slurries includes percent solids, pH, specific gravity, mean particle size and general (bulk) particle size distribution. However, a small number of “large” particles (>1 urn) have been found which fall outside of the specified size distribution. These particles can be aggregates, agglomerates or gels and may be formed from agglomeration, settling, system or pH shock or local drying of slurry. The large panicles and agglomerates can cause microscratches and they, together with the gels cause other defects on planarized wafer surfaces during CMP processing. Slurry filtration to remove these relatively large particles has proven to be beneficial in reducing wafer defects and increasing yields in CMP processes.
At the present time a wide variety of filter cartridge constructions are utilized to purify fluids. These cartridge constructions are designed to remove solids and colloidal particles as well as microorganisms. The basic two separate and distinct types of cartridges used in filtration of gases and liquids are depth filters (typically wound) and surface or screen filters (usually pleated). A depth filter is primarily used to remove most of the contaminants and particles. It is typically utilized upstream of a surface or screen filters. The most important properties for a depth filter are its “dirt holding capacity” or throughput, pressure drop and retention. The filter design allows contaminants and particles to be trapped in stages within the depth of the filter due to the construction of the multiple layers of various media types. A wound depth filter has multiple layers with the most open media (largest micron retention rating), i.e., largest pore size usually the outermost layer, adjacent the liquid inlet with the tightest media at the core adjacent the liquid outlet will have the least amount of surface area due to the smallest diameter around which it is wrapped. This layer at the core contributes to most of the pressure drop of the cartridge because the media has the highest pressure drop and the least amount of filtration surface area. Likewise, this layer will significantly reduce the capacity of the filter due to both the low filtration surface area and the smallest micron retention rating.
Presently available depth filters are positioned within a housing, spaced apart from the interior housing walls thereby to form a void volume upstream of the depth filter. This spacing is effected to permit either the introduction of a fluid feed into the entire filter or the removal of the entire permeate from the filter. If this spacing were not maintained, fluid flow through the filter can be severely restricted. As a result, a relatively large high fluid hold-up volume occurs in a conventional filter unit. A depth filter construction utilizing such a spacing also is disadvantageous for filtering a slurry since the particles in the slurry can settle out of the slurry on and within the filter. This results in rapid plugging of the filter, particularly at low flow rate point of use applications.
Depth filters comprising a relatively deep bed of filter material are undesirably compressible under the pressure of fluid entering the filter bed. Filter bed compressibility depends upon the type of filter, retention characteristics of the filter and thickness of the depth filter. For example, thicker filter beds are more compressible than thinner filter beds. When the filter bed is compressed, void volume is reduced and, the probability of plugging is increased. This results in an undesirably shod useful file of the filter. In addition, compression of the filter bed requires that the pressure of feed fluid be increased in order to maintain desired fluid through put rates. These conditions of increased pressure increase the probability of undesirable channelling of fluid in the space between the filter housing and the filter bed. Such channelling is undesirable since the channelled fluid does not pass through the filter bed and undesirably large particles are not removed from the fluid.
A surface of screen filter will retain virtually 100% of the particles or contaminants for which it is rated. The media used in surface or screen filter typically has a high pressure drop and low “dirt holding capacity” or throughput because of its high retention efficiency. The media normally used in a surface filter comprises glass or polymeric microfibers. Particles are retained by size exclusion primarily on the surface of the screen filter rather than within the depth of the filter. Particles smaller than the controlled pore size tend to be trapped within the media of the surface filter. However, as a result of the controlled pore structure, they provide more predictable filtration than depth filters. Screen filters are not useful for filtering a slurry since they will become plugged quickly by the solid particles and gels in the slurry.
Accordingly, it would be desirable to provide a filter cartridge including a depth filter for filtering a slurry that effectively removes undesirably large solid particles and gels. In addition, it would be desirable to provide such a filter cartridge that permits passage there through of particles in the slurry within a desired particle size range. Furthermore, it would be desirable to provide such a filter cartridge wherein compression of the depth filter is controlled to substantially prevent compression of the depth filter and channelling of the slurry being filtered.